@crazyg Correct, and not only for flags. TripleA comes with a full set of all required assets for a quantity of players and units. So, as long as your game has only those players and those units, you need no main assets at all.
Tho, I'm not sure how much of that is wanted, or it is just a legacy item that just so happen not having been touched in the moment all preinstalled maps have been removed?
Do the developers consider bad practice to duplicate assets in your maps that are available in the installation's assets (like if the map you mentioned would have those flags)?

To expand on above explanations. If the game located in C:\username/triplea/downloadedMaps/*gamename*/map is lacking an image or sound file. It will look in the following location. Which is the TripleA engine's installed location.

The typical install location for the triplea engine and the "assets" folders are below.